Human blood plasma is comprised of a variety of proteins that carry out a variety of functions. The protein components of blood plasma are a focus of intense research. See, for example, Anderson et al., Molecular & Cellular Proteomics, 3.4:311-326 (2004); and Ping et al, Proteomics, 5:3506-3519 (2005) for a description of the known protein components of human blood plasma. The most common protein found in human blood plasma is albumin.
Human albumin, also referred to as serum albumin, is a multifunctional protein found in blood plasma. It is an important factor in the regulation of plasma volume and tissue fluid balance through its contribution to the colloid osmotic pressure of plasma. Albumin also functions as a carrier for other molecules found in the bloodstream. Albumin normally constitutes 50-60% of plasma proteins and because of its relatively low molecular weight (66,500 Daltons), exerts 80-85% of the colloidal osmotic pressure of the blood. Albumin regulates transvascular fluid flux and hence, intra and extravascular fluid volumes, and transports lipid and lipid-soluble substances. Albumin solutions are frequently used for plasma volume expansion and maintenance of cardiac output in the treatment of certain types of shock or impending shock including those resulting from burns, surgery, hemorrhage, or other trauma or conditions in which a circulatory volume deficit is present.
Albumin has a blood circulation half-life of approximately two weeks and is designed by nature to carry other molecules such as lipids, peptides, and other proteins. A hydrophobic binding pocket and a free thiol cysteine residue (Cys34) are features that enable this function. Due to its low pKa (approx. 7) Cys34 is one of the more reactive thiol groups appearing in human plasma. The Cys34 of albumin also accounts for the major fraction of thiol concentration in blood plasma (over 80%) (Kratz et al., J. Med. Chem., 45(25):5523-33 (2002)). The ability of albumin through its reactive thiol to act as a carrier has been utilized for therapeutic purposes. For example, attachment of drugs to albumin to improve the pharmacological properties of the drugs has been described (Kremer et al., Anticancer Drugs, 13:(6):615-23 (2002); Kratz et al., J. Drug Target., 8(5):305-18 (2000); Kratz et al., J. Med. Chem., 45(25):5523-33 (2002); Tanaka et al., Bioconjug. Chem., 2(4):261-9 (1991); Dosio et al., J. Control. Release, 76(1-2):107-17 (2001); Dings et al., Cancer Lett., 194(1):55-66 (2003); Wunder et al., J. Immunol., 170(9):4793-801 (2003); Christie et al., Biochem. Pharmacol., 36(20):3379-85 (1987)). The attachment of peptide and protein therapeutics to albumin has also been described (Holmes et al., Bioconjug. Chem., 11 (4):439-44 (2000), Leger et al., Bioorg. Med. Chem. Lett., 13(20):3571-5 (2003); Paige et al., Pharm. Res., 12(12):1883-8 (1995)). Conjugates of albumin and interferon-alpha (Albuferon™) and of albumin and human growth hormone (Albutropin™) and of albumin and interleukin-2 (Albuleukin™) have also been made. The art also describes the use of standard recombinant molecular biology techniques to generate an albumin-protein fusion (U.S. Pat. No. 6,548,653, which is incorporated by reference herein). All but the latter conjugates with albumin involve ex vivo conjugate formation with an exogenous albumin. Potential drawbacks to using exogenous sources of albumin are contamination or an immunogenic response.
In vivo attachment of therapeutic agents to albumin has also been described, where, for example, a selected peptide is modified prior to administration to allow albumin to bind to the peptide. This approach is described using dipeptidyl peptidase IV-resistant glucagon-like-peptide-1 (GLP-1) analogs (Kim et al., Diabetes, 52(3):751-9 (2003)). A specific linker ([2-[2-[2-maleimido-propionamido-(ethoxy)-ethoxy]-acetamide) was attached to an added carboxyl-terminal lysine on the peptide to enable a cysteine residue of albumin to bind with the peptide. Others have investigated attaching specific tags to peptides or proteins in order to increase their binding to albumin in vivo (Koehler et al., Bioorg Med. Chem. Lett., 12(20):2883-6 (2002); Dennis et al., J. Biol. Chem., 277(38):35035-35043 (2002)); Smith et al., Bioconjug. Chem., 12:750-756 (2001)). A similar approach has been used with small molecule drugs, where a derivative of the drug was designed specifically to have the ability to bind with a cysteine residue of albumin. For example, this pro-drug strategy has been used for doxorubicin derivatives where the doxorubicin derivative is bound to endogenous albumin at its cysteine residue at position 34 (Cys34; Kratz et al., J. Med. Chem., 45(25): 5523-33 (2002)). The in vivo attachment of a therapeutic agent to albumin has the advantage, relative to the ex vivo approach described above, in that endogenous albumin is used, thus obviating problems associated with contamination or an immunogenic response to the exogenous albumin. Yet, the prior art approach of in vivo formation of drug conjugates with endogenous albumin involves a permanent covalent linkage between the drug and the albumin. To the extent the linkage is cleavable or reversible, the drug or peptide released from the conjugate is in a modified form of the original compound.
Human serum albumin has been expressed in yeast host cells including Saccharomyces cerevisiae (Etcheverry et al., (1986) BioTechnology 8:726, and EPA 123 544), Pichia pastoris (EPA 344 459), and Kluyveromyces (Fleer et al., (1991) BioTechnology 9:968-975), and in E. coli (Latta et al., (1987) BioTechnology 5:1309-1314), which are incorporated by reference herein.
A naturally produced antibody (Ab) is a tetrameric structure consisting of two identical immunoglobulin (Ig) heavy chains and two identical light chains. Immunoglobulins are molecules containing polypeptide chains held together by disulfide bonds, typically having two light chains and two heavy chains. In each chain, one domain (V) has a variable amino acid sequence depending on the antibody specificity of the molecule. The other domains (C) have a rather constant sequence common to molecules of the same class.
The heavy and light chains of an Ab consist of different domains. Each light chain has one variable domain (VL) and one constant domain (CL), while each heavy chain has one variable domain (VH) and three or four constant domains (CH). Each domain, consisting of about 110 amino acid residues, is folded into a characteristic β-sandwich structure formed from two β-sheets packed against each other, the immunoglobulin fold. The VL domains each have three complementarity determining regions (CDR1-3) and the VH domains each have up to four complementarity determining regions (CDR1-4), that are loops, or turns, connecting β-strands at one end of the domains. The variable regions of both the light and heavy chains generally contribute to antigen specificity, although the contribution of the individual chains to specificity is not necessarily equal. Antibody molecules have evolved to bind to a large number of molecules by using randomized CDR loops.
Functional substructures of Abs can be prepared by proteolysis and by recombinant methods. They include the Fab fragment, which comprises the VH-CH1 domains of the heavy chain and the VL-CL1 domains of the light chain joined by a single interchain disulfide bond, and the Fv fragment, which comprises only the VH and VL domains, and the Fc portion which comprises the non-antigen binding region of the molecule. In some cases, a single VH domain retains significant affinity for antigen (Ward et al., 1989, Nature 341, 554-546). It has also been shown that a certain monomeric κ light chain will specifically bind to its antigen. (L. Masat et al., 1994, PNAS 91:893-896). Separated light or heavy chains have sometimes been found to retain some antigen-binding activity as well (Ward et al., 1989, Nature 341, 554-546).
Another functional substructure is a single chain Fv (scFv), comprised of the variable regions of the immunoglobulin heavy and light chain, covalently connected by a peptide linker (S-z Hu et al., 1996, Cancer Research, 56, 3055-3061). These small (Mr 25,000) proteins generally retain specificity and affinity for antigen in a single polypeptide and can provide a convenient building block for larger, antigen-specific molecules. The short half-life of scFvs in the circulation limits their therapeutic utility in many cases.
A small protein scaffold called a “minibody” was designed using a part of the Ig VH domain as the template (Pessi et al., 1993, Nature 362, 367-369). Minibodies with high affinity (dissociation constant (Kd) about 10−7 M) to interleukin-6 were identified by randomizing loops corresponding to CDR1 and CDR2 of VH and then selecting mutants using the phage display method (Martin et al., 1994, EMBO J. 13, 5303-5309).
Camels often lack variable light chain domains when IgG-like material from their serum is analyzed, suggesting that sufficient antibody specificity and affinity can be derived from VH domains (three or four CDR loops) alone. “Camelized” VH domains with high affinity have been made, and high specificity can be generated by randomizing only the CDR3.
An alternative to the “minibody” is the “diabody.” Diabodies are small bivalent and bispecific antibody fragments, having two antigen-binding sites. The fragments comprise a heavy-chain variable domain (VH) connected to a light-chain variable domain (VL) on the same polypeptide chain (VH-VL). Diabodies are similar in size to the Fab fragment. By using a linker that is too short to allow pairing between the two domains on the same chain, the domains are forced to pair with the complementary domains of another chain and create two antigen-binding sites. These dimeric antibody fragments, or “diabodies,” are bivalent and bispecific. See, P. Holliger et al., PNAS 90:6444-6448 (1993).
An antibody fragment includes any form of an antibody other than the full-length form. Antibody fragments herein include antibodies that are smaller components that exist within full-length antibodies, and antibodies that have been engineered. Antibody fragments include but are not limited to Fv, Fc, Fab, and (Fab′)2, single chain Fv (scFv), diabodies, triabodies, tetrabodies, bifunctional hybrid antibodies, CDR1, CDR2, CDR3, combinations of CDR's, variable regions, framework regions, constant regions, and the like (Maynard & Georgiou, 2000, Annu. Rev. Biomed. Eng. 2:339-76; Hudson, 1998, Curr. Opin. Biotechnol. 9:395-402).
CDR peptides and organic CDR mimetics have been made (Dougall et al., 1994, Trends Biotechnol. 12, 372-379). CDR peptides are short, typically cyclic, peptides which correspond to the amino acid sequences of CDR loops of antibodies. CDR loops are responsible for antibody-antigen interactions. CDR peptides and organic CDR mimetics have been shown to retain some binding affinity (Smyth & von Itzstein, 1994, J. Am. Chem. Soc. 116, 2725-2733). Mouse CDRs have been grafted onto the human Ig framework without the loss of affinity (Jones et al., 1986, Nature 321, 522-525; Riechmann et al., 1988).
In the body, specific Abs are selected and amplified from a large library (affinity maturation). The processes can be reproduced in vitro using combinatorial library technologies. The successful display of Ab fragments on the surface of bacteriophage has made it possible to generate and screen a vast number of CDR mutations (McCafferty et al., 1990, Nature 348, 552-554; Barbas et al., 1991, Proc. Natl. Acad. Sci. USA 88, 7978-7982; Winter et al., 1994, Annu. Rev. Immunol. 12, 433-455). An increasing number of Fabs and Fvs (and their derivatives) are produced by this technique. The combinatorial technique can be combined with Ab mimics.
A number of protein domains that could potentially serve as protein scaffolds have been expressed as fusions with phage capsid proteins. Review in Clackson & Wells, Trends Biotechnol. 12:173-184 (1994). Several of these protein domains have already been used as scaffolds for displaying random peptide sequences, including bovine pancreatic trypsin inhibitor (Roberts et al., PNAS 89:2429-2433 (1992)), human growth hormone (Lowman et al., Biochemistry 30:10832-10838 (1991)), Venturini et al., Protein Peptide Letters 1:70-75 (1994)), and the IgG binding domain of Streptococcus (O'Neil et al., Techniques in Protein Chemistry V (Crabb, L. ed.) pp. 517-524, Academic Press, San Diego (1994)). These scaffolds have displayed a single randomized loop or region. Tendamistat has been used as a presentation scaffold on the filamentous phage M13 (McConnell and Hoess, 1995, J. Mol. Biol. 250:460-470).
The Fc portion of an immunoglobulin, includes but is not limited to, an antibody fragment which is obtained by removing the two antigen binding regions (the Fab fragments) from the antibody. One way to remove the Fab fragments is to digest the immunoglobulin with papain protease. Thus, the Fc portion is formed from approximately equal sized fragments of the constant region from both heavy chains, which associate through non-covalent interactions and disulfide bonds. The Fc portion can include the hinge regions and extend through the CH2 and CH3 domains to the C-terminus of the antibody. Representative hinge regions for human and mouse immunoglobulins can be found in Antibody Engineering, A Practical Guide, Borrebaeck, C. A. K., ed., W.H. Freeman and Co., 1992, the teachings of which are herein incorporated by reference. The Fc portion can further include one or more glycosylation sites. The amino acid sequences of numerous representative Fc proteins containing a hinge region, CH2 and CH3 domains, and one N-glycosylation site are well known in the art.
There are five types of human immunoglobulin Fc regions with different effector functions and pharmacokinetic properties: IgG, IgA, IgM, IgD, and IgE. IgG is the most abundant immunoglobulin in serum. IgG also has the longest half-life in serum of any immunoglobulin (23 days). Unlike other immunoglobulins, IgG is efficiently recirculated following binding to an Fc receptor. There are four IgG subclasses G1, G2, G3, and G4, each of which has different effector functions. G1, G2, and G3 can bind C1q and fix complement while G4 cannot. Even though G3 is able to bind C1q more efficiently than G1, G1 is more effective at mediating complement-directed cell lysis. G2 fixes complement very inefficiently. The C1q binding site in IgG is located at the carboxy terminal region of the CH2 domain.
All IgG subclasses are capable of binding to Fc receptors (CD16, CD32, CD64) with G1 and G3 being more effective than G2 and G4. The Fc receptor binding region of IgG is formed by residues located in both the hinge and the carboxy terminal regions of the CH2 domain.
IgA can exist both in a monomeric and dimeric form held together by a J-chain. IgA is the second most abundant Ig in serum, but it has a half-life of only 6 days. IgA has three effector functions. It binds to an IgA specific receptor on macrophages and eosinophils, which drives phagocytosis and degranulation, respectively. It can also fix complement via an unknown alternative pathway.
IgM is expressed as either a pentamer or a hexamer, both of which are held together by a J-chain. IgM has a serum half-life of 5 days. It binds weakly to C1q via a binding site located in its CH3 domain. IgD has a half-life of 3 days in serum. It is unclear what effector functions are attributable to this Ig. IgE is a monomeric Ig and has a serum half-life of 2.5 days. IgE binds to two Fc receptors which drives degranulation and results in the release of proinflammatory agents.
Polypeptides of the present invention may contain any of the isotypes described above or may contain mutated Fc regions wherein the complement and/or Fc receptor binding functions have been altered, modified, or removed. Polypeptides of the present invention may contain any of the isotypes described above or may contain mutated Fc regions wherein the effector function has been altered, modified, or removed. Thus, the polypeptides of the present invention may contain the entire Fc portion of an immunoglobulin, fragments of the Fc portion of an immunoglobulin, or analogs thereof.
Polypeptides of the present invention can consist of single chain proteins or as multi-chain polypeptides. Two or more Fc proteins can be produced such that they interact through disulfide bonds that naturally form between Fc regions. These multimers can be homogeneous with respect to a conjugated molecule or they may contain different conjugated molecules at the N-terminus of the Fc portion of the fusion protein.
A Fc or Fc-like region may serve to prolong the in vivo plasma half-life of a compound fused to it. Since the Fc region of IgG produced by proteolysis has the same in vivo half-life as the intact IgG molecule and Fab fragments are rapidly degraded, it is believed that the relevant sequence for prolonging half-life reside in the CH2 and/or CH3 domains. Further, it has been shown in the literature that the catabolic rates of IgG variants that do not bind the high-affinity Fc receptor or C1q are indistinguishable from the rate of clearance of the parent wild-type antibody, indicating that the catabolic site is distinct from the sites involved in Fc receptor or C1q binding. [Wawrzynczak et al., (1992) Molecular Immunology 29:221]. Site-directed mutagenesis studies using a murine IgG1 Fc region suggested that the site of the IgG1 Fc region that controls the catabolic rate is located at the CH2-CH3 domain interface. Fc regions can be modified at the catabolic site to optimize the half-life of the fusion proteins. The Fc region used for the fusion proteins of the present invention may be derived from an IgG1 or an IgG4 Fc region, and may contain both the CH2 and CH3 regions including the hinge region.
Chimeric molecules comprising Fc and one or more other molecules including, but not limited to, a polypeptide may be generated. The chimeric molecule can contain specific regions or fragments of Fc and the other molecule(s). Any such fragments can be prepared from the proteins by standard biochemical methods, or by expressing a polynucleotide encoding the fragment. A polypeptide, or a fragment thereof, can be produced as a fusion protein comprising human serum albumin (HSA), Fc, or a portion thereof. Fusions may be created by fusion of a polypeptide with a) the Fc portion of an immunoglobulin; b) an analog of the Fc portion of an immunoglobulin; and c) fragments of the Fc portion of an immunoglobulin.
Recently, an entirely new technology in the protein sciences has been reported, which promises to overcome many of the limitations associated with site-specific modifications of proteins. Specifically, new components have been added to the protein biosynthetic machinery of the prokaryote Escherichia coli (E. coli) (e.g., L. Wang, et al., (2001), Science 292:498-500) and the eukaryote Sacchromyces cerevisiae (S. cerevisiae) (e.g., J. Chin et al., Science 301:964-7 (2003), Drabkin et al., (1996) Mol. Cell. Biol., 16:907) and in mammalian cells (Sakamoto et al., (2002) Nucleic Acids Res. 30:4692), which has enabled the incorporation of non-genetically encoded amino acids to proteins in vivo. A number of new amino acids with novel chemical, physical or biological properties, including photoaffinity labels and photoisomerizable amino acids, photocrosslinking amino acids (see, e.g., Chin, J. W., et al. (2002) Proc. Natl. Acad. Sci. U.S.A. 99:11020-11024; and, Chin, J. W., et al., (2002) J. Am. Chem. Soc. 124:9026-9027), keto amino acids, heavy atom containing amino acids, and glycosylated amino acids have been incorporated efficiently and with high fidelity into proteins in E. coli and in yeast in response to the amber codon, TAG, using this methodology. See, e.g., J. W. Chin et al., (2002), Journal of the American Chemical Society 124:9026-9027; J. W. Chin, & P. G. Schultz, (2002), ChemBioChem 3(11):1135-1137; J. W. Chin, et al., (2002), PNAS United States of America 99:11020-11024; and, L. Wang, & P. G. Schultz, (2002), Chem. Comm., 1:1-11. All references are incorporated by reference in their entirety. These studies have demonstrated that it is possible to selectively and routinely introduce chemical functional groups, such as ketone groups, alkyne groups and azide moieties, that are not found in proteins, that are chemically inert to all of the functional groups found in the 20 common, genetically-encoded amino acids and that may be used to react efficiently and selectively to form stable covalent linkages.
The ability to incorporate non-genetically encoded amino acids into proteins permits the introduction of chemical functional groups that could provide valuable alternatives to the naturally-occurring functional groups, such as the epsilon —NH2 of lysine, the sulfhydryl —SH of cysteine, the imino group of histidine, etc. Certain chemical functional groups such as carbolyl, alkyne, and azide moieties described herein are known to be inert to the functional groups found in the 20 common, genetically-encoded amino acids but react cleanly and efficiently to form stable linkages.